Well, promotions are more or less voluntary incentives given out by the airline. As long as they promise the bonus miles either in writing or orally (problem of proof), they are legally bound to credit your account. Once the promise is gone (e.g. no longer a notice on their website), you as a customer donīt have a claim for these bonus miles.
If they credit you by mistake, this does not give you the right to be "mistakenly credited" the next time. The only thing you can do is just give it a try with customer service and see whether you get them. As far as legal steps, I donīt see any chance for success.